A driver circuit is commonly used to provide current or voltage to induce functionality in an actuator or transmitting device. For example, a laser driver circuit can be used to control the transmission of light from the laser diode. Optical communication involves the transmission of information from one place to another by sending a modulated light source, typically through an optical fiber cable. A laser diode is commonly used to transmit data in digital form over a telecommunications network. The light forms a carrier wave that is modulated to carry information. The laser diode requires high voltage swings from the driver circuit.
The laser driver may use metal oxide semiconductor field effect transistors (MOSFET) or complementary metal oxide semiconductor (CMOS) in a cascode arrangement to handle the high voltage swings. The source of a higher cascode MOSFET is connected to the drain of a lower cascode MOSFET, with substantially the same current flowing through both MOSFETs. In a differential configuration, the driver circuit has complementary outputs with a constant current (IMOD) steered alternatively between the two outputs in response to a differential data signal. The modulation current IMOD can be relatively large, up to 100 mA or more. Switching speeds for a gigabit driver circuit must be fast, e.g., tens of picoseconds. Therefore, the rate of change of current at the driver outputs (dI/dt) is very large during current switching transitions.
The DC resistance of the laser diode is typically small, often less than 10 ohms. However, the physical wiring between the driver and laser diode tends to have a parasitic inductive component Lp, which increases the effective impedance of the load at high frequencies and causes large peak transient voltage spikes at the driver output (V=Lp*dI/dt) during current switching transitions. The magnitude of the voltage spikes may damage, shorten the life, or otherwise adversely affect reliability of the MOSFETs.
In the prior art, MOSFETs have been implemented with a thick gate oxide to tolerate the high voltage spikes. However, a thick oxide MOSFET is inherently slower than small-geometry, thin-oxide MOSFETs due to longer minimum channel length, larger width to length ratio to achieve a given transconductance, and higher capacitance that must be charged and discharged each cycle of operation. The thick gate oxide MOSFETs are often unsatisfactory for high speed applications.